In a large pot or Instant Pot on sauté mode, cook the bacon until crispy. Remove and set aside.
Add the chopped onion and garlic to the pot and cook in the bacon drippings until translucent.
Add the sweet potatoes, chestnuts, and broth to the pot. Bring to a boil, then reduce to a simmer and cook until sweet potatoes are tender.
Blend the soup until smooth, using an immersion blender or transferring to a blender.
Stir in fresh sage and season with salt and pepper. Top with crispy bacon pieces and serve warm.